The present invention relates to a method for measuring the properties of a stack that viscous molten salt forms on the bottom of a soda recovery unit while black liquor is burned therein, and for measuring the corrosion of the materials of construction of the unit during the operation thereof.
In general, the properties of the stack and the corrosion of materials are determined by using known electrochemical measuring methods, for instance to measure the potential, polarization resistance, impedance and resistivity, together with the temperature.
The soda recovery unit, in which black liquor formed in the cooking step of sulphate pulp is burned, produces electric energy (about 40% of the total requirement) and steam needed in a pulp plant. On the other hand, the chemicals necessary for pulp cooking are regenerated in the soda recovery unit. As the black liquor is burned in the unit, a stack of viscous molten salt is formed on the bottom thereof. The control of this stack is of paramount importance to the balance and stability of the process, which in turn are prerequisites for the effective energy production and regeneration of chemicals. In addition, any changes in the stack have an influence on the service life of the piping on the bottom of the soda recovery unit.
In a pulp plant, the soda recovery unit is the most expensive, and probably the most critical unit, and thus the greatest safety risk. Inappropriate operation of the unit for any reason affects the whole plant. Irregular burning in the soda recovery unit leads to a decreased steam production, and due to this, the lacking amount of steam must be produced with an auxiliary unit. Because insufficient amounts of steam, and accordingly electricity are produced, more electricity has to be purchased from outside the plant. Process failures in the soda recovery unit interfere with the regeneration of the chemicals and increase air pollution. All these factors decrease directly or indirectly the productivity of the pulp plant. If the operation failure is severe enough to make the running down of the soda recovery unit necessary, then the pulp plant as a whole must be run down. For the plant, the economic losses due to this may amount each day on average to USD 300.000.
More serious than operation failures is an eventual damage of the piping under the stack, at the bottom of the unit. Normally, a solidified salt layer protects the bottom piping, but the properties of this protective layer may vary, or the layer may be absent altogether, for instance due to a shutdown of the unit or a process failure. The bottom piping of the unit may then be exposed to the molten salt. Molten salt is extremely corrosive, damaging and even giving rise to holes through the walls of the bottom piping. Water enters the stack through these holes, causing the risk of a so called melt water explosion that may damage the whole unit and cause economic losses summing up even to several millions of US dollars due to mere material damage, without considering production losses at all. In addition, the explosion may be so violent that even casualties are possible.
In the future, process conditions are becoming increasingly complicated due to more and more restrictive requirements concerning industrial production, recycling, and environment. The result has already been an increased corrosion in the soda recovery unit. Repairing the unit, and accordingly production losses due to extraordinary, or prolonged shutdowns, as well as additional inspection and maintenance may cost the plant several millions or tens of millions of dollars yearly. Recently, of particular concern has been the cracking of the bottom piping made of so called compound material. It is known that in Finland about 50% of the bottom piping made of this compound material have cracks. This cracking has been studied worldwide for several years without learning any actual reasons for it. Indeed, the bottle neck of the production of a pulp plant may in the future be the soda recovery unit which may not be operated with full capacity because of inadequate knowledge about the behavior of the stack and corrosion of the bottom piping material in these new process conditions.
At present, no system allowing the monitoring of the properties of the stack by direct measurements is available for controlling the soda recovery unit. Moreover, there are no apparatus for estimating the resistance of the construction materials and studying them in the actual environment in the unit. For the time being, the stack may only be monitored with cameras provided in the fire chamber, and with pyrometers measuring the temperature of the outer surface of the stack, but they do not give any data from within the stack. The bottom piping of the unit may at present be equipped with thermoelements for measuring the temperature, and the piping may be coated with a paint reacting to temperature changes, but, however, the information obtained with these methods is not sufficient as such to control the process, or especially to study the resistance of the materials of construction.
Indeed, the state of the art provides neither a suitable method, nor an apparatus for measuring the properties of the stack in a soda recovery unit, or studying and testing the resistance of materials in the actual conditions prevailing in the unit.
The object of the present invention is to provide a method for improving the controllability and safety of a soda recovery unit. More particularly, the object of the invention is to provide a method for measuring the properties of the stack in the soda recovery unit and the corrosion of construction materials thereof during operation, this method further taking advantage of the measured data to control directly or indirectly the soda recovery process.